The current design trend for connectors used in the computer field is to provide both high area density and high reliability connections between various circuit devices within a computer. Typically, integrated circuit chips are attached to a chip carrier, thermally conductive module chip carrier, circuit card, or board by solder bonding, brazing, controlled collapse chip connection (C4), wire lead bonding, metal bump bonding, tape automated bonding, or the like. High reliability for such connections is essential due to potential end product failure should poor connections of these devices occur.
There are two levels of interconnections that are typically required for electronic modules used for integrated circuit applications. The first level of interconnection is between the electronic module or substrate and the integrated circuit or chip.
The second level of interconnection is between the electronic module and a printed circuit board (PCB) or card. This is usually accomplished by a soldering process, relying on a lead metal alloy to provide both electrical and mechanical connection to the PCB or card.
For many applications the area density of the connections between the module and the PCB is low and the size of the module is only slightly larger than the chip. For these applications solder interconnections are usually acceptable. When modules become larger than about 32 mm as is the case of multi-chip modules (MCMs) which have more than one integrated circuit device and many inputs/outputs (I/Os), the reliability of solder joints for second level interconnection is compromised.
Connecting electrical terminations between the surfaces of two components, such as a ceramic chip carrier module and a glass-epoxy printed circuit board by solder requires a significant amount of compliance in the connector. This requirement is due to the non-planarity and irregularities inherent in the surfaces of the board and the module and, more importantly, their different coefficients of thermal expansion.
A solder joint is established by the strategic placement of solder balls, columns or the like between circuit elements (e.g., pads), and reflowing the solder to effect interconnection. During the cool down of the joined assembly from the reflow temperature, the solder connections at the outer most positions (typically the corners of the connection array), undergo shear stress due to differences between coefficients of thermal expansion of the module and the PCB.
Moreover, during use of the completed device, ambient heating or device generated heat causes uneven expansion of the board and the module, thereby leading to further stressing of the solder joint. The connector must accommodate this stress without severing the mechanical and electrical connection between the module and PCB. It should be noted that as the module size gets larger, the size of the interconnection array usually also increases. The mismatch due to differing coefficients of thermal expansion and the resultant shear stress in the solder also increases. For example, as the interconnection array size increases to greater than about 32 mm (1.25 inches), the stress on the outermost interconnections increases. This can sometimes be mitigated by using long solder connections between the module and the PCB as is practiced in solder column or wire column interconnection. These types of interconnects are often fragile and easily damaged during handling.
LGAs are electrical contacts located on an electrical device, which are used to provide electrical connections to the wiring within the device. The LGA typically consists of an array of metal pads, deposited through thick or thin film deposition techniques, and usually coated by plating with gold to provide non-oxidizing and highly conductive surfaces. Connections are made to these pads by metal or metallized connectors that physically contact the pads without the use of solders. The contacts between the LGA pads and the connectors are maintained by a force exerted by compressing the device with the LGA pads against the connectors. These connectors are typically arranged in an array within a socket so that a large number of electrical contacts can be made. A major advantage of LGA socket connectors is that they can be used to allow replacement of the electrical device quickly without the need to unsolder, remove, and resolder the device.
A variety of structures are used to connect the LGA pads on an electrical device to the metallized connectors. The connector used for LGAs is often made to connect two mating LGA pads positioned on each side of each connector. In this configuration the connector array is often referred to as an interposer since it is placed between opposing LGA features. Typically, interconnection features use a plurality of contacts held in a polymer laminate on centers complimentary to pads of components and PCB circuits. These contacts range from those requiring an extremely low closure force, such as those made of a conductive gel, to those requiring an intermediate force which are formed of fine conductive wire termed "fuzz" buttons and, for more rigorous applications, a type of coil spring known as a "canted" coil spring. The housings for the different applications clamp the contacts against the pads and are, accordingly, of different constructions, dependent upon the environment, vibration, and stress of the connectors.
An alternative connection method may involve the placement of connections directly on a surface by solder or electrically conductive epoxy. The solder or epoxy connects only one end of the connector to a PCB, for instance. The other end of the connector may be available to provide contact to an LGA pad and module, substrate or a different PCB.
The most widely used form of LGA socket (in this case, an array of connectors that contact opposing LGA pads) is the fuzz button connector, which is a polymer sheet that a very fine diameter molybdenum wire with numerous coatings, the last of which is predominantly gold. The connector is entangled and compressed, much like a steel wool pad, and then press-fit into the holes of the polymer sheet. Each connector is discretely formed and there is a tendency for a connector to fall out of the hole. Considerable force is required to maintain the contact between each connector and the LGA pad. There is also a very limited contact area between the fine diameter wire and the LGA pads. The connector size presents a problem because the pitch between pads needed for increasing Input/Output (I/O) density on a substrate requires very small, but accurately placed, connectors. The processes of handling and repairing small, discrete spring connectors approaching 15 to 20 mils in diameter is difficult at best. Consequently, an integrated connector array is required.
Specific examples that teach the use of discrete springs to effect interconnection between pads and metallized connectors may be seen in U.S. Pat. No. 5,139,427 issued to Boyd et al. on Aug. 18, 1992, and in a sales brochure by Servometer Corporation of Cedar Grove, N.J. The sales brochure teaches the use of discrete bellows for electrical connections. These bellows are manufactured from electrodeposited nickel and gold and are designed to provide force repeatability. Each bellows is a discretely formed cylindrical structure ranging in diameter from 0.037 inches to 0.125 inches outside diameter. At one end of the bellows, a contact having a convex conical tip is formed. At the other end, a contact having a convex conical receptacle is formed. Each bellows is discretely mounted on a substrate surface by inserting the receptacle end onto a pin and press fitting it into place. Alternatively, the receptacle end may be spot welded to a pin or soldered into place.
Discrete bellows used for an electrical array may have the same problems as the discrete fuzz buttons. The use of discrete bellows in high-density I/O applications may not be feasible. The processes of handling and repairing discrete small springs approaching 15 to 20 mils in diameter is impractical and very expensive. Therefore, a need exists for a process and structure which increases reliability and decreases the complexity of fabrication of the connection between an area array package and a supporting substrate.